1. Field of the Invention
The present invention relates to a near-field optical scanning microscope having an extremely high resolution.
2. Description of the Related Art
When matter of sub-micron size is viewed through the normal optical microscope, this viewing is limited because of light diffraction and the positional resolution of matter to an extent smaller than about .lambda./2 has been regarded as being impossible. In the case of a near-field microscope wherein light is shot to a sample at a position separated from the sample surface by several nm and light thus passed through or reflected from the sample is detected by a photomultiplier tube, while using evanescent light irradiated through a fine pinhole, however, it has been made possible to view any pattern having a size of about 0.1 .mu.m.
FIG. 45 shows the near-field scanning microscope reported by U. During et al. A sample is arranged in an xy-scanner system driven by a bimorph and a probe for radiating light to the sample is arranged adjacent to the sample in the case of this microscope. The probe has an aperture at the tip thereof and the evanescent light is shot from the probe through this aperture. FIG. 46 shows logarithmic strengths of evanescent light relative to distances measured from the aperture at the tip of the probe. Normalized distance .zeta. obtained by dividing a distance (z) of the sample measure from the aperture at the tip of the probe by a diameter (a) of the aperture is plotted on the axis of abscissas. The relation of light strengths relative to the normalized distance .zeta. can be shown by three areas (which correspond to PROX, NEAR AND FAR in FIG. 46) and when .zeta.=0.3-1 (which corresponds to an actual distance of 2-5 nm), the strength of evanescent light becomes substantially certain. In the case of the near-field microscope shown in FIG. 45, the distance between the sample and the probe is set to be at the area of PROX. Therefore, the sample is conductive and the position of the probe is controlled in a direction (z) in such a way that tunnel current flowing between the conductive sample and the probe becomes certain.
The evanescent light entering into the sample through the tip of the probe is condensed by an objective and amplified by a photomultiplier tube. Light including optical information on the sample surface is condensed in this manner by the objective. When the sample is xy-scanned while keeping the distance between the probe and the sample at the area of PROX, the optical image of near-field can be obtained. The positional resolution of the near-field optical microscope is determined by the size of the aperture at the tip of the probe, the sensibility of the detectors used, as well as other factors, and it is expected that a resolution of about 20 nm can be actually obtained.
Matter having a size of sub-micron order can be viewed only by a technique which uses electrons such as SEM and TEM, and these techniques need a vacuum space and information obtained by these techniques is electrical in nature. Recently, other techniques of using tunnel current such as STM and AFM and atomic force are being practiced. Sub-micron observation by light can be carried out even in the air and any matter such as the human body and cells, difficult to be viewed, can be made objects for this observation. Therefore, this sub-micron observation by light can be applied to a wide range of fields.
In the case of the near-field microscope, piezo elements and bimorphs used by STM and other techniques can be used to position the probe in the direction (z) and the scanner system in directions (x) and (y), but it is difficult to make the probe through which the evanescent light is irradiated.
According to the above-described near-field microscopes, probes made by the following manner are used. As shown in FIG. 47A, an aluminium film is vapor-deposited on a quartz chip whose tip is made sharp by machining. The aluminium film thus deposited is then pressed against the sample at the tip portion thereof and crushed to form an aperture at the crushed tip portion. The probe made by this manner has a life of only several hours because the size of the aperture at its tip cannot be made certain and the diameter of the aperture is increased while it is being used (it is supposed that this is caused by the internal stress of aluminium).